Add upstream address and name tags (#104)

When a request is redirected to an upstream, then it is not visible
where it went. The ngx_http_upstream module maintains a variable
"upstream_addr" for the resolved IP address + port information. It
also sets the name of the used upstream config entry to the request
as request->upstream->upstream->host.
So add that information to the new tags "upstream.address" and
"upstream.name".

opentracing/src/ngx_http_opentracing_module.cpp

@@ -50,6 +50,7 @@ const std::pair<ngx_str_t, ngx_str_t> default_opentracing_tags[] = {
     {ngx_string("component"), ngx_string("nginx")},
     {ngx_string("nginx.worker_pid"), ngx_string("$pid")},
     {ngx_string("peer.address"), ngx_string("$remote_addr:$remote_port")},
+    {ngx_string("upstream.address"), ngx_string("$upstream_addr")},
     {ngx_string("http.method"), ngx_string("$request_method")},
     {ngx_string("http.url"), ngx_string("$scheme://$http_host$request_uri")},
     {ngx_string("http.host"), ngx_string("$http_host")}};

opentracing/src/request_tracing.cpp

@@ -67,6 +67,19 @@ static void add_status_tags(const ngx_http_request_t *request,
   }
 }
 
+//------------------------------------------------------------------------------
+// add_upstream_name
+//------------------------------------------------------------------------------
+static void add_upstream_name(const ngx_http_request_t *request,
+                              opentracing::Span &span) {
+  if (!request->upstream || !request->upstream->upstream ||
+      !request->upstream->upstream->host.data)
+    return;
+  auto host = request->upstream->upstream->host;
+  auto host_str = to_string(host);
+  span.SetTag("upstream.name", host_str);
+}
+
 //------------------------------------------------------------------------------
 // RequestTracing
 //------------------------------------------------------------------------------
@@ -157,6 +170,7 @@ void RequestTracing::on_exit_block(
     add_script_tags(main_conf_->tags, request_, *span_);
     add_script_tags(loc_conf_->tags, request_, *span_);
     add_status_tags(request_, *span_);
+    add_upstream_name(request_, *span_);
 
     // If the location operation name is dependent upon a variable, it may not
     // have been available when the span was first created, so set the operation
@@ -184,6 +198,7 @@ void RequestTracing::on_log_request() {
                  "finishing opentracing request span for %p", request_);
   add_status_tags(request_, *request_span_);
   add_script_tags(main_conf_->tags, request_, *request_span_);
+  add_upstream_name(request_, *request_span_);
 
   // When opentracing_operation_name points to a variable and it can be
   // initialized or modified at any phase of the request, so set the